How to make a physics simulation permanent
First off, go to the target frame and make sure only your pebbles are selected.

In the 3D Viewport, under Object > Rigid Body select Apply Transformation. This will make the current position the pebbles' starting one.



Then, in the same submenu, click Remove to remove the pebbles from the Rigid Body simulations; that is, to make them simple, static meshes again.